Methods and apparatus for conducting financial transactions

ABSTRACT

A method of establishing wireless communications between an interrogator and individual ones of multiple wireless identification devices, the method comprising combining tree search and Aloha methods to establish communications between the interrogator and individual ones of the multiple wireless identification devices without collision. A system comprising an interrogator, and a plurality of wireless identification devices configured to communicate with the interrogator. In a wireless fashion, the respective wireless identification devices having a unique identification number, the interrogator being configured to employ tree search and Aloha techniques to determine the unique identification numbers of the different wireless identification devices so as to be able to establish communications between the interrogator identification and individual devices without ones of the multiple wireless collision by multiple wireless identification devices attempting to respond to the interrogator at the same time.

PRIORITY AND RELATED APPLICATIONS

This patent application is a continuation of and claims priority to U.S.patent application Ser. No. 13/275,157 filed Oct. 17, 2011, entitled“Method and Apparatus for Conducting Financial Transactions”, which is acontinuation of and claims priority to U.S. patent application Ser. No.11/855,860 filed Sep. 14, 2007, entitled “Method of Addressing Messagesand Communications Systems”, which is a continuation of and claimspriority to U.S. patent application Ser. No. 11/416,846 filed May 2,2006, entitled “Method of Addressing Messages and Communication System”,now U.S. Pat. No. 7,639,638, which is a continuation of and claimspriority to U.S. patent application Ser. No. 09/820,467 filed Mar. 28,2001, entitled “Method of Addressing Messages and CommunicationsSystems”, now U.S. Pat. No. 7,315,522, which is a continuation of andclaims priority to U.S. patent application Ser. No. 09/026,248 filedFeb. 19, 1998, entitled “Method of Addressing Messages andCommunications Systems”, now U.S. Pat. No. 6,275,476, each of theforegoing incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to communications protocols and to digital datacommunications. Still more particularly, the invention relates to datacommunications protocols in mediums such as radio communication or thelike. The invention also relates to radio frequency identificationdevices for inventory control, object monitoring, determining theexistence, location or movement of objects, or for remote automatedpayment.

BACKGROUND OF THE INVENTION

Communications protocols are used in various applications. For example,communications protocols can be used in electronic identificationsystems. As large numbers of objects are moved in inventory, productmanufacturing, and merchandising operations, there is a continuouschallenge to accurately monitor the location and flow of objects.Additionally, there is a continuing goal to interrogate the location ofobjects in an inexpensive and streamlined manner. One way of trackingobjects is with an electronic identification system.

One presently available electronic identification system utilizes amagnetic coupling system. In some cases, an identification device may beprovided with a unique identification code in order to distinguishbetween a number of different devices. Typically, the devices areentirely passive (have no power supply), which results in a small andportable package. However, such identification systems are only capableof operation over a relatively short range, limited by the size of amagnetic field used to supply power to the devices and to communicatewith the devices.

Another wireless electronic identification system utilizes a largeactive transponder device affixed to an object to be monitored whichreceives a signal from an interrogator. The device receives the signal,then generates and transmits a responsive signal. The interrogationsignal and the responsive signal are typically ‘radio-frequency (RF)signals produced by an RF transmitter circuit.

Because active devices have their own power sources, and do not need tobe in close proximity to an interrogator or reader to receive power viamagnetic coupling. Therefore, active transponder devices tend to be moresuitable for applications requiring tracking of a tagged device that maynot be in close proximity to an interrogator. For example, activetransponder devices tend to be more suitable for inventory control ortracking.

Electronic identification systems can also be used for remote payment.For example, when a radio frequency identification device passes aninterrogator at a toll booth, the toll booth can determine the identityof the radio frequency identification device, and thus of the owner ofthe device, and debit an account held by the owner for payment of tollor can receive a credit card number against which the toll can becharged. Similarly, remote payment is possible for a variety of othergoods or services.

A communication system, such as a wireless identification system,typically includes two transponders: a commander station orinterrogator, and a responder station or transponder device whichreplies to the interrogator.

If the interrogator has prior knowledge of the identification number ofa device which the interrogator is looking for, it can specify that aresponse IS requested only from the device with that identificationnumber. Sometimes, such information is not available. For example, thereare occasions where the interrogator is attempting to determine which ofmultiple devices are within communication range.

When the interrogator sends a message to a transponder device requestinga reply, there is a possibility that multiple transponder devices willattempt to respond simultaneously, causing a collision, and thus anerroneous message to be received by the interrogator. For example, ifthe interrogator sends out a command requesting that all devices withina communications range identify themselves, and gets a large number ofsimultaneous replies, the interrogator may not able to interpret any ofthese replies. Thus, arbitration schemes are employed to permitcommunications free of collisions.

In one arbitration scheme or system, described in commonly assigned U.S.Pat. Nos. 5,627,544; 5,583,850; 5,500,650; and 5,365,551, all toSnodgrass et al. and all incorporated herein by reference, theinterrogator sends a command causing each device of a potentially largenumber of responding devices to select a random number from a knownrange and use it as that device's arbitration number. By transmittingrequests for identification to various subsets of the full range ofarbitration numbers, and checking for an error-free response, theinterrogator determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator is able to conduct subsequent uninterrupted communicationwith devices, one at a time, by addressing only one device.

Another arbitration scheme is referred to as the Aloha or slotted Alohascheme. This scheme is discussed In various references relating tocommunications, such as Digital Communications: Fundamentals andApplications, Bernard Sklar, published January 1988 by Prentice Hall. Inthis type of scheme, a device will respond to an interrogator using oneof many time domain slots selected randomly by the device. A problemwith the Aloha scheme is that if there are many devices or potentially Nmany devices in the field (i.e. in communications range, capable ofresponding) then there must be many available slots or many collisionswill occur. Having many available slots slows down replies. If themagnitude of the number of devices in a field is unknown, then manyslots are needed. This result in the system slowing down significantlybecause the reply time equals the number of slots multiplied by the timeperiod required for one reply.

An electronic identification system which can be used as a radiofrequency identification device, arbitration schemes, and variousapplications for such devices are described in detail in commonlyassigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29,1996, and incorporated herein by reference.

SUMMARY OF THE INVENTION

The invention provides a wireless identification device configured toprovide a signal to identify the device in response to an interrogationsignal.

One aspect of the invention provides a method of establishing wirelesscommunications between an interrogator and individual ones of multiplewireless identification devices. Tree search and Aloha methods arecombined to establish communications between the interrogator andindividual ones of the multiple wireless identification devices withoutcollision.

One aspect of the invention provides a method of addressing messagesfrom an interrogator to a selected one or more of a number ofcommunications devices. A first predetermined number of bits areestablished to be used as unique identification numbers. Uniqueidentification numbers respectively having the first predeterminednumber of bits are established for respective devices. A secondpredetermined number of bits are established to be used for randomvalues. The devices are caused to select random values. Respectivedevices choose random values independently of random values selected bythe other devices. The interrogator transmits a command requestingdevices having random values within a specified group of random valuesto respond, the specified group being less than or equal to the entireset of random values. Devices receiving the command respectivelydetermine if their chosen random values fall within the specified groupand, if so, send a reply to the interrogator within a randomly selectedtime slot of a number of slots. If not, they do not send a reply. Theinterrogator determines if a collision occurred between devices thatsent a reply and, if so, creates a new, smaller, specified group.

One aspect of the invention provides a communications system comprisingan interrogator, and a plurality of wireless identification devicesconfigured to communicate with the interrogator in a wireless fashion.The respective wireless identification devices have a uniqueidentification number. The interrogator is configured to employ treesearch and Aloha techniques to determine the unique identificationnumbers of the different wireless identification devices so as to beable to establish communications between the interrogator and individualones of the multiple wireless identification devices without collisionby multiple wireless to identification devices attempting to respond tothe interrogator at the same time.

Another aspect of the invention provides a system comprising aninterrogator configured to communicate to a selected one or more of anumber of communications devices, and a plurality of communicationsdevices. The devices are configured to select random values. Respectivedevices choose random values independently of random values selected bythe other devices. The interrogator is configured to transmit a commandrequesting devices having random values within a specified group ofrandom values to respond, the specified group being less than or equalto the entire set of random values. Devices receiving the command areconfigured to respectively determine if their chosen random values fallwithin the specified group and, if so, send a reply to the interrogatorwithin a randomly selected time slot of a number of slots. If not, theydo not send a reply. The interrogator is configured to determine if acollision occurred between devices that sent a reply and, if so, createa new, smaller, specified group.

One aspect of the invention provides a radio frequency identificationdevice comprising an integrated circuit including a receiver, atransmitter, and a microprocessor. In one embodiment, the integratedcircuit is a monolithic single die single metal layer integrated circuitincluding the receiver, the transmitter, and the microprocessor. Thedevice of this embodiment includes an active transponder, instead of atransponder which relies on magnetic coupling for power, and thereforehas a much greater range.

One aspect of the invention provides a method of conducting a financialtransaction via radio frequency communications.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below /2 withreference to the following accompanying drawings.

FIG. 1 is a high level circuit schematic showing an interrogator and aradio frequency identification device embodying the invention.

FIG. 2 is a front view of a housing, In the form of a badge or card,supporting the circuit of FIG. 1 according to one embodiment theinvention.

FIG. 3 is a front view of a housing supporting the circuit of FIG. 1according to another embodiment of the invention.

FIG. 4 is a diagram illustrating a tree splitting sort method forestablishing communication with a radio frequency identification devicein a field of a plurality of such devices, without collisions.

FIG. 5 is a time line plot illustrating operation of a slotted Alohascheme.

FIG. 6. is a diagram illustrating using a combination of a treesplitting sort method with an Aloha method for establishingcommunication with a radio frequency identification device in a field ofa plurality of such devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIG. 1 illustrates a wireless identification device 12 in accordancewith one embodiment of the invention. In the illustrated embodiment, thewireless identification device is a radio frequency data communicationdevice 12, and includes RFID circuitry 16. In the illustratedembodiment, the RFID circuitry is defined by an integrated circuit asdescribed in the above-incorporated patent application Ser. No.08/705,043, filed Aug. 29, 1996. Other embodiments are possible. A powersource 18 is connected to the integrated circuit 16 to supply power tothe integrated circuit 16. In one embodiment, the power source 18comprises a battery. The device 12 further includes at least one antenna14 connected to the circuitry 16 for wireless or radio frequencytransmission and reception by the circuitry 16.

The device 12 transmits and receives radio frequency communications toand from an interrogator 26. An exemplary interrogator is described inU.S. patent application Ser. No. 08/907,689, filed Aug. 8, 1997 andincorporated herein by reference. Preferably, the interrogator 26includes an antenna 28, as well as dedicated transmitting and receivingcircuitry, similar to that implemented on the integrated circuit 16.

Generally, the interrogator 26 transmits an interrogation signal orcommand 27 via the antenna 28. The device 12, receives the incominginterrogation signal via its antenna 14. Upon receiving the signal 27,the device 12 responds by generating and transmitting a responsivesignal or reply 29. The responsive signal 29 typically includesinformation that uniquely identifies, or labels the particular device 12that is transmitting, so as to identify any object or person with whichthe device 12 is associated.

Although only one device 12 is shown in FIG. 1, typically there will bemultiple devices 12 that correspond with the interrogator 26, and theparticular devices 12 that are in communication with the interrogator 26will typically change over time. In the illustrated embodiment In FIG.1, there is no communication between multiple devices 12. Instead, thedevices 12 respectively communicate with the interrogator 26. Multipledevices 12 can be used in the same field of an interrogator 26 (i.e.,within communications range of an interrogator 26). Similarly, multipleinterrogators 26 can be in proximity to one or more of the devices 12.

The radio frequency data communication device 12 can be 6 included inany appropriate housing or packaging. Various methods of manufacturinghousings are described in commonly assigned U.S. patent application Ser.No. 08/800,037, filed Feb. 13, 1997, and incorporated herein byreference.

FIG. 2 shows but one embodiment in the form of a card or badge 19including the radio frequency data communication device 12, and ahousing 11 including plastic or other suitable material. In oneembodiment, the front face of the badge has visual identificationfeatures such as graphics, text, information found on identification orcredit cards, etc.

FIG. 3 illustrates but one alternative housing supporting the device 12.More particularly, FIG. 3 shows a miniature housing 20 encasing thedevice 12 to define a tag which can be supported by an object (e.g.,hung from an object, affixed to an object, 20 etc.). Although twoparticular types of housings have been disclosed, the device 12 can beincluded in any appropriate housing.

If the power source 18 is a battery, the battery can take any suitableform. Preferably, the battery type will be selected depending on weight,size, and life requirements for a particular application. In oneembodiment, the battery 18 is at thin profile or button-type cellforming a small, thin energy cell more commonly utilized in watches andsmall electronic devices requiring a thin profile. A conventional cellhas a pair of electrodes, an anode formed by one face and a cathodeformed by an opposite face. In an alternative embodiment, the powersource 18 comprises a series connected pair of cells. Instead of using abattery, any suitable power source can be employed.

The circuitry 16 further includes a backscatter transmitter and isconfigured to provide a responsive signal to the interrogator 26 byradio frequency. More particularly, the circuitry 16 includes atransmitter, a receiver, and memory such as is described in U.S. patentapplication Ser. No. 08/705,043.

Radio frequency identification has emerged as a viable and affordablealternative to tagging or labeling small to large quantities of items.The interrogator 26 communicates with the devices 12 via an RF link, soall transmissions by the interrogator 26 are heard simultaneously by alldevices 12 within range.

If the interrogator 26 sends out a command requesting that all devices12 within range identify themselves, and gets a large number ofsimultaneous replies, the interrogator 1.6 may not be able to interpretany of these replies. Therefore, arbitration schemes are provided.

If the interrogator 26 has prior knowledge of the identification numberof a device 12 which the interrogator 26 is looking for, it can specifythat a response is requested only from the device 12 with thatidentification number. To target a command at a specific device 12,(i.e., to initiate point-on-point communication), the interrogator 26must send a number identifying a specific device 12 along with thecommand. At start-up, or in a new or changing environment, theseidentification numbers are not known by the interrogator 26. Therefore,the interrogator 26 must identify all devices 12 in the field (withincommunication range) such as by determining the identification numbersof the devices 12 in the field. After this is accomplished,point-to-point communication can proceed as desired by the interrogator26.

Generally speaking, RFID systems are a type of multi-accesscommunication system. The distance between the interrogator 26 anddevices 12 within the field is typically fairly short (e.g., severalmeters), so packet transmission time is determined primarily by packetsize and baud rate. Propagation delays are negligible. In RFID systems,there is a potential for a large number of transmitting devices 12 andthere is a need for the interrogator 26 to work in a changingenvironment, where different devices 12 are swapped in and outfrequently (e.g., as inventory is added or removed). The inventors havedetermined that, in such systems, the use of random access methods workeffectively for contention resolution (i.e., for dealing with collisionsbetween devices 12 attempting to respond to the interrogator 26 at thesame time).

RFID systems have some characteristics that are different from othercommunications systems. For example, one characteristic of theillustrated RFID systems is that the devices 12 never communicatewithout being prompted by the interrogator 26. This is in contrast totypical multi-access systems where the transmitting units operate moreindependently. In addition, contention for the communication medium isshort lived as compared to the ongoing nature of the problem in othermulti-access systems. For example, in a RFID system, after the devices12 have been identified, the interrogator can communicate with them in apoint-to-point fashion.

Thus, arbitration In a RFID system is a transient rather thansteady-state phenomenon. Further, the capability of a device 12 islimited by practical restrictions on size, power, and cost. The lifetimeof a device 12 can often be measured in terms of number of transmissionsbefore battery power is lost. Therefore, one of the most importantmeasures of system performance in RFID arbitration is total timerequired to arbitrate a set of devices 12. Another measure is powerconsumed by the devices 12 during the process. This is in contrast tothe measures of throughput and packet delay in other types ofmulti-access systems.

FIG. 4 illustrates one arbitration scheme that can be employed forcommunication between the interrogator and devices 12. Although thearbitration system is being described in connection with a wirelessidentification system or RFID system, this and other arbitration schemesdisclosed herein can be employed in any communication system. Generally,the interrogator 26 sends a command causing each device 12 of apotentially large number of responding devices 12 to select a randomnumber from a known range and use it as that device's arbitrationnumber. By transmitting requests for identification to various subsetsof the full range of arbitration numbers, and checking for an error-freeresponse, the interrogator 26 determines the arbitration number of everyresponder station capable of communicating at the same time. Therefore,the interrogator 26 is able to conduct subsequent uninterruptedcommunication with devices 12, one at a time, by addressing only onedevice 12.

Three variables are used: an arbitration value (AVALUE), an arbitrationmask (AMASK), and a random value ID (RV). The interrogator sends acommand causing each device of a potentially large number of respondingdevices to select a random number from a known range and use it as thatdevice's arbitration number. The interrogator sends an arbitration value(AVALUE) and an arbitration mask (AMASK) to a set of devices 12. Thereceiving devices 12 evaluate the following equation: (AMASK &AVALUE)==(AMASK & RV) wherein “&” is a bitwise AND function, and wherein“==” is an equality function. If the equation evaluates to “1” (TRUE),then the device 12 will reply. If the equation evaluates to “0” (FALSE),then the device 12 will not reply. By performing this In a structuredmanner, with the number of bits in the arbitration mask being increasedby one each time, eventually a device 12 will respond with nocollisions. Thus, a binary search tree methodology is employed.

An example using actual numbers will now be provided using only fourbits, for simplicity, reference being made to FIG. 4. In one embodiment,sixteen bits are used for AVALUE and AMASK,

respectively. Other numbers of bits can also be employed depending, forexample, on the number of devices 12 expected to be encountered in aparticular application, on desired cost points, etc.

Assume, for this example, that there are two devices 12 in the field,one with a random value (RV) of 1100 (binary), and another with a randomvalue (RV) of 1010 (binary). The interrogator is trying to establishcommunications without collisions being caused by the two devices 12attempting to communicate at the same time.

The interrogator sets AVALUE to 0000 (or all “don't care”, indicated bythe character “X” in FIG. 4) and AMASK to 0000. The interrogatortransmits a command to all devices 12 requesting that they identifythemselves. Each of the devices 12 evaluate (AMASK & AVALUE)==(AMASK &RV) using the random value RV that the respective devices 12 selected.If the equation evaluates to “1” (TRUE), then the device 12 will reply.If the equation evaluates to “0” (FALSE), then the device 12 will notreply. In the first level of the illustrated tree, AMASK is 0000 andanything bitwise ANDed with all zeros results in all zeros, so both thedevices 12 in the field respond, and there is a collision.

Next, the interrogator sets AMASK to 0001 and AVALUE to 0000 andtransmits an identify command. Both devices 12 in the field have a zerofor their least significant bit, and (AMASK & AVALUE)==(AMASK & RV) willbe true for—both devices 12. For the device 12 with a random value of1100, the left side of the equation is evaluated as follows (0001 &0000)=0000. The right side is evaluated as (0001 & 1100)=0000. The leftside equals the right side, so the equation is true for the device 12with the random value of 1100. For the device 12 with a random value of1010, the left side of the equation is evaluated as (0001 & 0000)=0000.The right side is evaluated as (0001 & 1010)=0000. The left side equalsthe right side, so the equation is true for the device 12 with therandom value of 1010. Because the equation is true for both devices 12in the field, both devices 12 in the field respond, and there is anothercollision.

Recursively, the interrogator next sets AMASK to 0011 with AVALUE stillat 0000 and transmits an identify command. (AMASK & AVALUE)==(AMASK &RV) is evaluated for both devices 12. For the device 12 with a randomvalue of 1100, the left side of the equation is evaluated as (0011 &1010)=0000. The right side is evaluated as (0011 & 1100)=0010. The leftside equals the right side, so the equation is true for the device 12with the random value of 1100, so this device 12 responds. For thedevice 12 with a random value of 1010, the left side of the equation isevaluated as (0011 & 0000)=0000. The right side is evaluated as (0011 &1010)=0010. The left side does not equal the right side, so the equationis false for the device 12 with the random value of 1010, and thisdevice 12 does not respond. Therefore, there is no collision, and theinterrogator can determine the identity (e.g., an identification number)for the device 12 that does respond.

De-recursion takes place, and the devices 12 to the right for the sameAMASK level are accessed by setting AVALUE at 0010 and using the sameAMASK value 0011.

The device 12 with the random value of 1010 receives a command andevaluates the equation (AMASK & AVALUE)==(AMASK & RV). The left side ofthe equation is evaluated as (0011 & 0010)=0010. The right side of theequation is evaluated as (0011 & 1010)=0010. The right side equals theleft side, so the equation is true for the device 12 with the randomvalue of 1010. Because there are no other devices 12 in the subtree, agood reply is returned by the device 12 with the random value of 1010.There is no collision, and the interrogator can determine the identity(e.g., an identification number) for the device 12 that does respond.

By recursion, what is meant 1S that a function makes a call to itself.In other words, the function calls itself within the body of thefunction. After the called function returns, derecursion takes place andexecution continues at the place just after the function call; i.e. atthe beginning of the statement after the function call.

For instance, consider a function that has four statements (numbered1,2,3,4) in it, and the second statement is a recursive call. Assumethat the fourth statement is a return statement. The first time throughthe loop (iteration 1) the function executes the statement 2 and(because it is a recursive call) calls itself causing iteration 2 tooccur. When iteration 2 gets to statement 2, it calls itself makingiteration 3. During execution in iteration 3 of statement 1, assume thatthe function does a return. The information that was saved on the stackfrom iteration 2 is loaded and the function resumes execution atstatement 3 (in iteration 2), followed by the execution of statement 4which is also a return statement. Since there are no more statements inthe function, the function de-recursion to iteration 1. Iteration 1, hadpreviously recursively called itself in statement 2. Therefore, it nowexecutes statement 3 (in iteration 1). Following that it executes areturn at statement 4. Recursion is known in the art.

Consider the following code, which employs recursion, and which can beused to implement operation of the method shown in FIG. 4 and describedabove.

Arbitrate(AMASK, AVALUE)   {   collision=IdentifyCmnd(AMASK, AVALUE)  if (collision) then    {     /* recursive call for left side */   Arbitrate((AMASK<<1)+1, AVALUE)     /* recursive call for right side*/    Arbitrate((AMASK<<1)+1, AVALUE+(AMASK+1))   }  /* endif */  } /*return */

The symbol “<<” represents a bitwise left shift. “<<1” means shift leftby one place. Thus, 0001<<1 would be 0010. Note, however, that AMASK isoriginally called with a value of zero, and 0000<<1 is still 0000.Therefore, for the first recursive call, AMASK=(AMASK<<1)+1. So for thefirst recursive call, the value of AMASK is 0000+000 1=000 1. For thesecond call, AMASK=(0001<<1)+1=0010+1=0011. For the third recursivecall, AMASK=(0011<<1)+1=0110+1=0111.

The routine generates values for AMASK and AVALUE to be used by theinterrogator in an identify command “IdentifyCmnd.” Note that theroutine calls itself if there IS a collision. Derecursion occurs whenthere is no collision. AVALUE and AMASK would have values such as thefollowing assuming there are collisions all the way down to the bottomof the tree.

AVALUE AMASK 0000 0000 0000 0001 0000 0011 0000 0111 0000 1111* 10001111* 0100 0111 0100 1111* 1100 1111*

This sequence of AMASK, AVALUE binary numbers assumes that there arecollisions all the way down to the bottom of the tree, at which pointthe Identify command sent by the interrogator

is finally successful so that no collision occurs. Rows in the table forwhich the interrogator is successful in receiving a reply withoutcollision are marked with the symbol “*”. Note that if the Identifycommand was successful at, for example, the third line in j the tablethen the interrogator would stop going down that branch of the tree andstart down another, so the sequence would be asshown in the following table.

AVALUE AMASK 0000 0000 0000 0001 0000 0011* 0010 0011 . . . . . .

This method is referred to as a splitting method. It works by splittinggroups of colliding devices 12 into subsets that are resolved in turn.The splitting method can also be viewed as a type of tree search. Eachsplit moves the method one level deeper in the tree. Either depth-firstor breadth first traversals of the tree can be employed.

Another arbitration method that can be employed is referred to as the“Aloha>>method. In the Aloha method, every time a device 12 is involvedin a collision, it waits a random period of time before retransmitting.This method can be improved by dividing time into equally sized slotsand -forcing transmissions to be aligned with one of these slots. Thisis referred to as “slotted Aloha.” In operation, the interrogator asksall devices 12 in the field to transmit their identification numbers inthe next time slot.

If the response is garbled, the interrogator informs the devices 12 thata collision has occurred, and the slotted Aloha scheme is put intoaction. This means that each device 12 in the field responds 12 withinan arbitrary slot determined by a randomly selected value. In otherwords, in each successive time slot, the devices 12 decide to transmittheir identification number with a certain probability.

The Aloha method is based on a system operated by the University ofHawaii. In 1971, the University of Hawaii began operation of a systemnamed Aloha. A communication satellite was used to interconnect severaluniversity computers by use of a random access protocol. The systemoperates as follows. Users or devices transmit at any time they desire.After transmitting, a user listens for an acknowledgment from thereceiver or interrogator. Transmissions from different users willsometimes overlap in time (collide), causing reception errors in thedata in each of the contending messages. The errors are detected by thereceiver, and the receiver sends a negative acknowledgment to the users.When a negative acknowledgment is received, the messages areretransmitted by the colliding users after a random delay. If thecolliding users attempted to retransmit without the random delay, theywould collide again. If the user does not receive either anacknowledgment or a negative acknowledgment within a certain amount oftime, the user “times out” and retransmits the message.

In the slotted Aloha scheme, a sequence of coordination pulses isbroadcast to all stations (devices). As is the case with the pure Alohascheme, packet lengths are constant. Messages are required to be sent ina slot time between synchronization pulses, and can be started only atthe beginning of a time slot. This reduces the rate of collisionsbecause only messages transmitted in the same slot can interfere withone another. The retransmission mode of the pure Aloha scheme ismodified for slotted Aloha such that if a negative acknowledgmentoccurs, the device retransmits after a random delay of an integer numberof slot times.

FIG. 5 illustrates operation of the slotted Aloha scheme. FIG. 5 shows apacket of data bits transmitted by a first device 12 a, which issubstantially identical to the device 12. The interrogator

26 acknowledges receipt without collision, as indicated in FIG. 5 by thesymbol ACK. FIG. 5 also shows devices 12 b and 12 c, also substantiallyidentical to the device 12, simultaneously transmitting24 packets of data to the interrogator 26, resulting in a collision. Theinterrogator returns a negative acknowledgment, as indicated in FIG. 5by the symbol NAK. The devices 12 band 12 c then respectively selectrandom numbers, and retransmit after a time delay corresponding to theselected random number. There is a possibility that the devices 12 b and12 c will again transmit at the same times, causing another collision,but in that case they will retransmit again using newly selected randomnumbers until there is no collision.

Another form of Aloha scheme is called reservation-Aloha. Thereservation-Aloha system has two basic modes: an unreserved mode, and areserved mode.

In the unreserved mode, a time frame IS established and divided into anumber of small reservation subslots. Users (devices) use these subslotsto reserve message slots. After requesting a reservation, the user(device) listens for an acknowledgment and a slot assignment.

In the unreserved mode, a time frame is divided into a certain number ofslots whenever a reservation is made. All but the last slot are used formessage transmissions. The last slot is subdivided into subs lots to beused for reservations. Users (devices) send message packets in theirassigned portions of the slots reserved for message transmissions.

FIG. 6 illustrates combining a tree sort method of a type such as theone shown in FIG. 4 with an Aloha method. Combining the two methodsallows a minimal number of slots to be used and takes advantage of theconquer and divide approach of the tree sort method. The method shown InFIG. 6, proceeds in a manner similar to the manner described inconnection with FIG. 4, except that devices 12 in the field that replyfor the given AMASK and AVALUE, reply within a randomly selected timeslot. This significantly reduces the number of collisions. In oneembodiment, the reply includes the unique identification number of theparticular device 12. In one embodiment, the reply includes the randomvalue RV selected by the particular device 12. In one embodiment, thereply includes both the unique identification number of the particulardevice 12 as well as the random value RV selected by the same device 12.

In one embodiment, the same randomly selected time slot is used by adevice 12 at different levels of the tree (i.e., for different values ofAMASK and AVALUE). In another embodiment, different randomly selectedtimes slots are used by a device 12 at different levels of the tree(i.e., for different values of AMASK and AVALUE). In one embodiment, acombination of these approaches is used. For example, one embodimentutilizes a method where the interrogator goes down the tree until someresponses without collision are received, before the devices 12re-randomize their Aloha random number. This can be classified as anadaptive method. Other adaptive methods are possible. For example, inone embodiment, the number of Aloha slots is reduced at lower levels ofthe tree. The number of slots can be reduced by the same number for eachlevel down the tree, or by a number

that varies depending on the number of levels down the tree. Thus, forexample, the number of slots can remain constant through a progressiondown the tree until some responses without collision are received, atwhich point the number of slots is reduced.

Thus, this embodiment provides the advantages of both the Aloha methodsand the tree sorting methods of establishing communications withoutcollisions.

In another embodiment, levels of the search tree are skipped. Skippinglevels in the tree, after a collision caused by multiple devices 12responding reduces the number of subsequent collisions without addingsignificantly to the number of no replies. In real-time systems, it isdesirable to have quick arbitration sessions on a set of devices 12whose unique identification numbers are unknown. Level skipping reducesthe number of collisions, both reducing arbitration time and conservingbattery life on a set of devices 12. In one embodiment, every otherlevel is skipped. In alternative embodiments, more than one level isskipped each time.

The trade off that must be considered in determining how many (if any)levels to skip with each decent down the tree is as follows. Skippinglevels reduces the number of collisions, thus saving battery power inthe devices 12. Skipping deeper (skipping more than one level) furtherreduces the number of collisions. The more levels that are skipped, thegreater the reduction in collisions. However, skipping levels results inlonger search times because the number of queries (Identify commands)increases. The more levels that are skipped, the longer the search timesSkipping just one level has an almost negligible effect on search time,but drastically reduces the number of collisions. If more than one levelis skipped, search time increases substantially. Skipping every otherlevel drastically reduces the number of collisions and saves batterypower without significantly increasing

/J the number of queries.

Level skipping methods are described in a commonly assigned patentapplication (attorney docket M140-117) naming Clifton W. Wood, Jr. andDon Hush as inventors, titled “Method of Addressing Messages, Method ofEstablishing Wireless Communications, and Communications System,” filedconcurrently herewith, and incorporated herein by reference.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1.-40. (canceled)
 41. A method of conducting a financial transaction viaradio frequency communications, the method comprising: operating a hostcomputer, the host computer having an application program forcontrolling an interrogator, the host computer configured to: cause thetransmission of a first wireless command from the interrogator toinitiate identification of one of a population of at least one radiofrequency communications device, the first wireless command includingfirst bits that are configured to elicit one or more responses from thepopulation; cause the interrogator to receive a first reply from the oneradio frequency communications device in response to the first wirelesscommand; cause the transmission of a second wireless command from theinterrogator to initiate identification of one of a subpopulation of atleast one radio frequency communications device, the second wirelesscommand including second bits that are configured to elicit one or moreresponses from the subpopulation; cause the interrogator to receive asecond reply from the one radio frequency communications device inresponse to the second wireless command; cause the identification of theidentifier of the one radio frequency communications device when thereis no collision in response to the second wireless command; and causethe transmission of a third wireless command to individually identifythe one radio frequency communications device using the identifier ofthe one radio frequency communications device, the third wirelesscommand requesting the one radio frequency communications device totransmit a subsequent reply; and cause the debiting of an accountassociated with the one radio frequency communications device based atleast in part on at least a portion of the replies from the one radiofrequency communications device.
 42. The method of claim 41, wherein theone radio frequency communications device is disposed within a card. 43.The method of claim 42, wherein the debiting of the account isassociated with the payment of a toll.
 44. The method of claim 43,wherein the interrogator is disposed within or proximate to a tollbooth, and the method further comprises operating the interrogatordisposed within the toll booth at least when the one radio frequencycommunications device issuing the reply to the first wireless command isin proximity to the toll booth.
 45. The method of claim 41, wherein thedebiting of the account comprises receiving data relating to a creditaccount against which a toll can be charged.
 46. The method of claim 45,wherein the data relating to the credit account comprises a credit cardnumber.
 47. The method of claim 42, wherein the debiting of the accountcomprises receiving data relating to a credit account which can becharged.
 48. The method of claim 47, wherein the data relating to thecredit account comprises a credit card number.
 49. The method of claim41, wherein the debiting of the account is for the payment of goods orservices.
 50. A radio frequency communications-based method ofconducting a financial transaction, comprising: operating a hostcomputer coupled to a master wireless device, the host computer havingan application program for controlling the master wireless device, thehost computer configured to: cause the transmission of a first wirelesscommand that is configured to elicit one or more responses from one ormore radio frequency communications devices within an operating field ofthe master wireless device, the first wireless command configured toelicit responses from a full set of arbitration values associated withthe one or more radio frequency communications devices; cause thetransmission of a second wireless command that is configured to elicitone or more responses from the one or more radio frequencycommunications devices within the operating field of the master wirelessdevice, the second wireless command configured to elicit responses froma subset of arbitration values associated with the one or more radiofrequency communications devices, the subset being less than the fullset; cause the master wireless device to receive a reply from one radiofrequency communications device in response to the first wirelesscommand and/or the second wireless command; cause the identification,from the reply, of an identifier of the one radio frequencycommunications device when there is no collision; and cause thetransmission of a third wireless command to individually address the oneradio frequency communications device using the identifier of the oneradio frequency communications device, the third wireless commandrequesting the one radio frequency communications device to transmit asubsequent reply; and receiving funds for the payment of goods orservices based at least on part on one or more replies from the oneradio frequency communications device.
 51. The method of claim 50,wherein the financial transaction is associated with the payment of atoll.
 52. The method of claim 51, wherein the master wireless device isdisposed within or proximate to a toll booth.
 53. The method of claim51, wherein the receiving of funds further comprises receiving relatingto a credit account against which the toll can be charged.
 54. Themethod of claim 53, wherein the data relating to the credit accountcomprises a credit card number.
 55. The method of claim 50, wherein theone radio frequency communications device is embodied within a card. 56.The method of claim 55, wherein the card includes visual identificationfeatures including at least one of graphics and/or text.
 57. A method ofconducting a financial transaction via radio frequency communications,comprising: debiting an account associated with a target radio frequencycommunications device, the debiting of the account enabled by a wirelessmaster device that is configured to: cause the transmission of a firstwireless command having first bits that are indicative of a full set ofarbitration values, the first wireless command configured to elicitresponses from a full set of radio frequency communications deviceswhich may be present within an operating field of the master wirelessdevice; cause the transmission of a second wireless command havingsecond bits that are indicative of a subset of arbitration values, thesecond wireless command configured to elicit responses from a subset ofthe radio frequency communications devices, the subset being less thanthe full set; receive a reply from the target radio frequencycommunications device in response to the first wireless command and/orthe second wireless command; cause the determination of, from the reply,an identifier of the target radio frequency communications device whenthere is no collision; and cause the transmission of a third wirelesscommand to individually address the target radio frequencycommunications device using the identifier.
 58. The method of claim 57,wherein the third wireless command requests that the target radiofrequency communications device transmit a subsequent reply.
 59. Themethod of claim 57, further comprising providing the target radiofrequency communications device to a user.
 60. The method of claim 59,wherein the target radio frequency communications device is disposedwithin a card.
 61. The method of claim 57, wherein the debiting of theaccount is associated with the payment of a toll.
 62. The method ofclaim 61, wherein the debiting of the account comprises receiving datacorresponding with a credit account against which a toll can be charged.63. The method of claim 62, wherein the data corresponding with thecredit account comprises a credit card number.
 64. The method of claim57, wherein the debiting of the account comprises receiving datacorresponding with a credit account which can be charged.
 65. The methodof claim 54, wherein the debiting of the account is for the payment ofgoods or services.
 66. A method of conducting a financial transactionvia radio frequency communications, the method comprising: debiting anaccount associated with a target radio frequency communications device,the debiting of the account enabled by an interrogator that isconfigured to: cause the transmission of a first wireless command fromthe interrogator to initiate identification of the target radiofrequency communications device within an operating field of theinterrogator, the first wireless command including first bits that areconfigured to elicit response from one or more devices within theoperating field; cause the interrogator to receive a first reply fromthe target radio frequency communications device in response to thefirst wireless command; cause the transmission of a second wirelesscommand from the interrogator to further identify the target radiofrequency communications device, the second wireless command includingsecond bits that are configured to elicit response from a subset of theone or more devices within the operating field; cause the interrogatorto receive a second reply from the target radio frequency communicationsdevice in response to the second wireless command; cause theidentification of the identifier of the target radio frequencycommunications device when there is no collision in response to thesecond wireless command; and cause the transmission of a third wirelesscommand to individually address the target radio frequencycommunications device using the identifier of the target radio frequencycommunications device.
 67. The method of claim 66, wherein the thirdwireless command requests the target radio frequency communicationsdevice to transmit a subsequent reply.
 68. The method of claim 66,wherein the target radio frequency communications device is disposedwithin a card, the card comprising visual identification featuresincluding at least one of graphics and/or text.
 69. The method of claim66, wherein: the debiting of the account is associated with the paymentof a toll; the interrogator is disposed within or proximate to a tollbooth; and the method further comprises operating the interrogator atleast when the target radio frequency communications device issuing thereply to the first wireless command is in proximity thereto.
 70. Themethod of claim 66, wherein: the debiting of the account comprisesreceiving data corresponding to a credit card account which can becharged; and the debiting of the account is for the payment of goods orservices.